Mating roles, copulation duration and reproductive output in the hermaphroditic freshwater snail Planorbella trivolvis

Hermaphrodites exhibit a wide variety of reproductive strategies, yet empirical studies lag behind theories about their evolution. Planorbella trivolvis, a simultaneous hermaphrodite capable of reciprocal copulation and rare self-fertilization, provides a unique model system for studying reproduction. This study assessed sex roles during mating, demonstrating that mating is primarily reciprocal, and investigated the relationship between physical coupling and reproductive output for single copulation events. Snails were isolated before sexual maturity and placed in 10-l aquaria in size-matched groups. Upon copulation, pairs were removed to small cups until they separated; they were then kept isolated and eggs counted until egg production ceased. The frequency of reciprocal copulation (both members of a pair produced egg masses following copulation) was 95% (35/37 productive matings), higher than previously reported. The duration of egg production ranged from 19 to 197 days (mean ± SE = 110.13 ± 6.3 days) with a sharp drop in production of egg masses and eggs per mass occurring between 15 and 20 weeks postmating, consistent with previous estimates of sperm storage. There was no relationship between mating duration and total egg production (a biomarker of sperm availability) or the duration of egg production (a marker of sperm longevity). Further investigation is needed to directly estimate sperm transfer and tease apart the duration of reciprocal physical contact from gamete and accessory protein transfer.